In commonly-used display devices, each pixel is composed of three subpixels for displaying three primaries of light, i.e., red, green, and blue, whereby multicolor display is achieved.
However, conventional display devices have a problem in that they can only display colors in a narrow range (called a “color gamut”). When the color gamut is narrow, some object colors (i.e., colors of various objects existing in nature; see Non-Patent Document 1) cannot be displayed. Therefore, in order to broaden the color gamut of a display device, there has been proposed a technique which increases the number of primary colors to be used for displaying.
For example, Patent Document 1 discloses a display device which performs display by using six primary colors. Patent Document 1 also discloses a display device which performs display by using four primary colors and a display device which performs display by using five primary colors. An example of a display device which performs display by using six primary colors is shown in FIG. 25. In a display device 800 shown in FIG. 25, one pixel P is composed of a red subpixel R, a green subpixel G, a blue subpixel B, a cyan subpixel C, a magenta subpixel M, and a yellow subpixel Ye. The display device 800 attains multicolor displaying by intermixing the six primary colors of red, green, blue, cyan, magenta, and yellow which are displayed by the six subpixels.
Increasing the number of primary colors to be used for displaying, i.e., performing display by using four or more primary colors, provides a broader color gamut than those of conventional display devices which perform display by using three primaries. In the present specification, display devices which perform display by using four or more primary colors will be referred to as “multiprimary display devices”, and display devices which perform display by using three primary colors (i.e., those which are conventional and commonly used) will be referred to as “three-primary display devices”.
However, given the same screen size, in order for a multiprimary display device to display an image having a similar resolution to that of a three-primary display device, the device structure needs to become finer, whereby the production cost is increased. The reason is as follows. When the number of subpixels per pixel increases from 3, to 4 or more in a multiprimary display device, the subpixel size needs to become smaller than that of a three-primary display device in order to obtain the same number of pixels in the same screen size. Specifically, given a number m of primary colors to be used for displaying (m≧4), the subpixel size must become 3/m. For example, in a multiprimary display device which performs display by using six primary colors, the subpixel size must be made ½(= 3/6).
Techniques for solving this problem are proposed in Patent Documents 2 and 3. In the multiprimary display devices disclosed in Patent Documents 2 and 3, the plurality of subpixels composing each pixel are regrouped into a plurality of imaginary pixels (called “virtual pixels”), and display is performed by regarding each of the plurality of virtual pixels is regarded as the smallest unit of multicolor displaying. As a result, even if the resolution of an input image is higher than the panel resolution, it is possible to suitably perform display.
Thus, the multiprimary display devices of Patent Documents 2 and 3 can provide a displaying resolution that is higher than the panel resolution, so that an image having a similar or higher resolution can be displayed with the same subpixel size and screen size as those of the three-primary display device. Moreover, they can be produced with a similar cost to that of a three-primary display device.
As a specific exemplary construction of multiprimary display devices capable of performing display with virtual pixels as described above, Patent Documents 2 and 3 disclose a construction where a signal conversion circuit for converting a three-primary image signal into a multiprimary image signal has a low-range multiprimary signal generation section, a high-range luminance signal generation section, and a rendering process section.
The low-range multiprimary signal generation section of this construction generates a low-range multiprimary signal based on an input image signal. The low-range multiprimary signal is a signal in which a low-range component of the input image signal has been adapted to multiprimaries. Based on the input image signal, the high-range luminance signal generation section generates a high-range luminance signal. The high-range luminance signal is a signal in which a high-range component of the input image signal has been subjected to luminance conversion. The rendering process section performs a rendering process onto a plurality of virtual pixels, based on the low-range multiprimary signal which has been generated by the low-range multiprimary signal generation section and the high-range luminance signal which has been generated by the high-range luminance signal generation section.
In the signal conversion circuit of the above construction, human visual characteristics are taken into account, i.e., there being higher sensitivity with respect to a luminance signal than with respect to a color signal (i.e., the luminous factor as to color differences is lower than the luminous factor as to luminance); thus, a multiprimary process is applied to the low-range component of an input image signal, whereas a luminance conversion process is applied to the high-range component. Then, the low-range multiprimary signal and the high-range luminance signal resulting from these processes are combined, and rendered onto virtual pixels, whereby an image signal (multiprimary image signal) corresponding to four or more primary colors is output.